What made the large exit hole?
Well....you figure that the bullet mushrooms and then starts pushing on stuff in it's way. The force of the material being pushed fans out over a larger area. Also remember that the bullet is no longer flying point end first but may have tumbled and is slowing down rapidly as the area of the bullet increases in diameter and the medium that it is in is very dense.
A half dollar sized slug that is tumbling and pushing is going to cut through much of the "meat" but will also be pushing quite a bit more mass than just the bullet itself.
Finally...the bullet reaches the other side of the animal and a blow-out occurs. The skin seperates and some of the matter and mass that had been accelerated comes out the other end about the same time that the bullet exits.
Hydrostatic shock? Hardly.
Simple physics? Yep.
